Lesson One
What are pollutants?
A pollutant is a toxic or harmful material released into the atmosphere.
What is chemical weathering?
All rocks at the Earth’s surface weather as a result of physical, chemical or
biological weathering. Chemical weathering occurs as certain chemicals,
attack the minerals in a rock. The chemical reactions that take place can
lead to dissolution of minerals or to the formation of new minerals. The most
common chemicals involved with chemical weathering are atmospheric
pollutants.
Which pollutants are associated with the chemical weathering of rocks?



Sulphur oxides
Nitrogen oxides
Carbon dioxide
All occur naturally.
However the level of pollutants found in the atmosphere has steadily
increased since the beginning of the industrial age, approximately 200 years
ago. The levels of sulphur oxides, nitrogen oxides and carbon dioxide in urban
areas can be mapped against human activity.
How do pollutants attack the surface of a rock?

Wet deposition
The gases may combine with water droplets in the atmosphere to produce an
acidic solution; when the water droplets are released, as rain, the rainwater
is slightly acidic.
Carbon dioxide and water combine to form carbonic acid
H2O + CO2  H2CO3
This is acid rain.

Dry deposition
The pollutant sulphur dioxide is often deposited on the rock surface in the
form of a gas, which then either dissolves in the moisture already present in
the stone or reacts with the calcium carbonate to produce a new mineral,
gypsum.
CaCO3 + SO2  CaSO3 CO2
This reaction also plays a role in the production of acid rain as the break
down of the calcium carbonate releases carbon dioxide.
Where do the pollutants come from?
Sources of pollution for wet deposition may be many miles away from the
building being attacked by the rain, the source of pollutants involved in dry
deposition are local. The following activities are responsible for most of the
pollution produced by human activity.


Burning fossil fuels for energy production
Emissions from car exhausts
Natural sources of the pollutants include
Volcanic eruptions
What effect does acid rain have on rock?
Acid rain reacts with the minerals in a rock; it reacts with rock composed of
the mineral calcite, which is formed from calcium carbonate (CaCO3). The
rocks are described as carbonate rocks.
Rocks formed from calcite are:
Limestone
Chalk
Marble
When carbonic acid reacts with calcite the following chemical process takes
place:
CaCo3 + H2CO3 
calcite
carbonic acid
(acid rain)
Ca + 2HCO3
calcium
bicarbonate ions
This is a chemical weathering process, the breakdown of rock in situ as a
result of chemical changes to the minerals.
What happens to the rock as a result of this process?
The calcium and bicarbonate ions are carried away in solution. The outer
layers of the rock are gradually lost as acid rain attacks the exposed surface.
Rainwater will also attack the mineral crystals within the rock if the rock is
sufficiently porous to allow water to seep into it.
Activity
 Question: What factors promote chemical weathering?

The type of pollutants dissolved in rainwater.

Rainwater containing dissolved carbon dioxide is slightly acidic. Acid
rain however has a pH range of between1.5 to 5.6. Therefore the more
sources of sulphur dioxide and nitrogen oxides there are in an area the
more chemical weathering there will be.
 Question: What other factors promote chemical weathering?
Design and carry out an experiment to test the following questions.
1. Is the rate of chemical weathering affected by temperature?
2. Is the rate of chemical weathering affected by surface area?
This experiment could be conducted using an indigestion tablet such as
Rennie with a calcium carbonate base, however they do not dissolve quickly
and if the water is agitated to speed up the breakdown it becomes too cloudy
to enable students to monitor the breakdown accurately. Using a tablet that
fizzes in water, such as Alka Seltzer provides a more vivid example of the
effects of changing the water temperature and surface area even though
this does not have a carbonate base.
 Use a whole class activity to design the experiment. Ask students to
predict what will happen if water temperature is increased or surface
area increased. What will they need to do to ensure the experiment is
controlled and results can be shared?
o Decide on the amount of water to be used so each test is the
same, say 100ml
o Use a control. One group uses water at room temperature.
o Ensure accurate measurement of water temperature
o Ensure accurate division of tablets – if crushing do this with a
pestle and mortar and ensure all grains are tipped into the
water, if breaking the tablet in half measure and cut accurately.
o Ensure accurate measurement of time. Students will use a
stopwatch but will need to agree how to time – start when the
tablet is submerged, stop when no bubbles are seen for example.
o Use a common recording format

In order to complete the experiments in one lesson give
each group a different water temperature to work with.
Results will then be shared amongst the whole class.
Experiments


Use whole tablets and different water temperatures (from iced
water to water at 45 degrees C) to determine if temperature
has an affect on chemical weathering rates

3 tests. Use a whole, a half and a crushed tablet dissolved in
water to determine if surface area is important to the rate of
chemical weathering. Each group carries out this experiment
using water at the same temperature as for their first
experiment.
Either use an interactive whiteboard, a computer or a ready
prepared OHT to collect the results. Have a table ready and ask
one member from each group to fill in their group’s results as they
complete the experiment. Print the results so each group has
access to the data.

Do the results match the predictions?
Homework
Each student will need a copy of the free publication Sustainable
Development Indicators In Your Pocket, 2005. These can be obtained in bulk
from DEFRA Publications, telephone 08459 556000 and quote code PB 11008
for A6 size booklets or PB 11008A for A4 size.
The section of the book dealing with air quality highlights the effect road
traffic is having on air quality. You know that certain air borne pollutants will
lead to the formation of acid rain.
Task
Using the graphs in this section of the booklet identify trends for sulphur
dioxide, nitrogen oxides and carbon dioxide and answer the following
question:
Should we still be worrying about acid rain?
Next find out if the air quality in your area is good or bad.
Use the web site http://www.airquality.co.uk/archive/index.php
Use the map and click on your regional area
This will tell you what the air quality is like where you live
Follow the link - For More Information - to find more detailed information
from the monitoring sites in your region – is there a monitoring site in your
village or town?
Task
Based on the information you have found about your local area from the web
site, not the information from the booklet, write another sentence or two to
answer the same question:
Should we still be worrying about acid rain?
Which of the resources you have used do you think was most useful for
helping you answer the question?
Alternative homework activity
What is air quality like in your area?
Each student will need a piece of laminated card with a hole punched near the
top. Thread a piece of string through the hole. Cover the card with doubled
sided sticky tape. The student chooses a place to hang the card, either in
their garden or in the school grounds. Students will need a sealable bag in
which to place their piece of card when it is collected. Cards should be
collected on the morning of the next lesson and brought to class. What
differences can be seen between the cards? Are the students surprised at
the amount of airborne pollution in their area? Were those cards paced
nearer the road dirtier than those placed in the school grounds?
What do the students think they could do to reduce levels of pollution?